1. Field of the Invention
The present invention relates to a MOS semiconductor device and a manufacturing method thereof. More particularly, the present invention relates to a MOS semiconductor device and a manufacturing method thereof characterized in contact with metal wiring.
2. Description of the Related Art
The gate length of a MOS transistor and the diameter of a contact hole are reduced as semiconductor integrated circuit device (LSI) are highly integrated. For example, when one LSI device is manufactured by a finer generation rule, the semiconductor device has been integrated by reducing an entire size thereof approximately in the same ratio with the same basic layout. There is no problem if all generation rules are reduced (linearly shrunk) in the same ratio in this way. However, all the generation rules cannot be linearly shrunk necessarily. As a result, manufacturing processes of the semiconductor device cannot be improved since no area for the semiconductor device can be sufficiently reduced.
When this problem is considered in relation to a contact hole, a diameter of the contact hole is very important as a generation rule. As shown in FIG. 1, overlapping margins M of fields and metal wiring around the contact hole 1 exist at any time. These margins cannot be reduced in the same ratio as the diameter of the contact hole so that high integration of the semiconductor device is prevented.
There is a so-called common contact technique for increasing an integration degree of contacts. With respect to this common contact technique, three contacts or more are connected to each other through one contact hole. FIG. 2a shows one example of this common contact. A diffusion region 4 of an N-type or a P-type having a high impurity concentration is formed on a substrate 3. A polysilicon electrode 6 and metal wiring 8 are connected to the diffusion region 4. In FIG. 2a, an insulating film 5 is formed between the substrate 3 and the polysilicon electrode 6. An insulating film 7 is formed between the polysilicon electrode 6 and the metal wiring 8.
When the common electrode is provided as in FIG. 2a, the diameter of a contact hole forming the common contact must be also reduced naturally in the same ratio as the other constructional portions in accordance with a generation rule. However, when three contacts or more are three-dimensionally connected to each other as in FIG. 2a, it is difficult to hold contact resistance to a sufficiently low level in comparison with normal contact resistance. When the size of a bottom face of the contact hole within the diffusion region 4 in FIG. 2a is set to d, contact resistance between the metal wiring 8 and the diffusion region 4 with respect to the size d is measured by a Kelvin method. FIG. 2b shows measured results of this contact resistance. As shown in FIG. 2b, when the size d is equal to or smaller than 0.4 .mu.m, the contact resistance Rc is rapidly increased.